1. Field of the Invention:
The present invention relates to a high resistance, low noise, high quantum efficiency photoconductor detector structure utilizing layers of photoconductive materials of predetermined band gaps operable to achieve high resistance detectors for use in multi-element detector arrays.
2. Description of the Prior Art:
The first operating infrared detection systems utilized detector arrays made of photoconductive mercury cadmium telluride. For example, these detectors consisted of a small slab of mercury cadmium telluride, 2 mils square by approximately 10 microns thick with two contacts one at each end of the photoconductor. The resistance of this material was approximately 50 to 100 ohms at 77.degree. Kelvin.
When exposed to infrared photons, the resistance of this mercury cadmium telluride photoconductive slab changed. The resistance change was measured as a voltage change across each delineated photoconductive detector located in the slab. Because of the complexity of the readout electronics and the power consumed by each detector, only a small number of detectors could be included in an array. Each detector had a small resistance, of approximately 50 to 100 ohms. An amplifier was necessary to read-out the signal from the detector. Implicitly, the size of photoconductive detector arrays were constrained by the power consumptions necessary for amplifiers required to read-out the low resistance detectors. For example, if each mercury cadmium telluride photoconductive detector consumed about a milliwatt of power, then a large focal plane array with a thousand detectors, required several watts of cryogenic cooling power. Such large cryogenic requirements were impractical.
The first generation of infrared focal plane arrays comprising 100 to 200 photoconductive mercury cadmium telluride detectors were accommodated with read-out electronics and focal plane cryogenic with cooling power capabilities of approximately one watt.
The problem to be solved then, is the development of a large infrared array for use in second generation systems that will overcome the limitations inherent in photoconductive low resistance detectors, by an approach that provides an IR array having low focal plane cooling power requirements and less complex read-out electronics.
Photoconductive mercury cadmium telluride has the beneficial feature of photoconductive gain. This photoconductive gain allows the reading of each detector even though the detector resistance is very low. If the power required to cool a photoconductive focal plane array must be reduced, then the complexity associated with the read-out circuitry and the power consumed by each detector must also be reduced. Both of these improvements can be achieved if the resistance of each individual detector is increased sufficiently to eliminate the need for high power read-out circuits. Replacement of high power consuming read-out circuits with low power read-out circuits (i.e., charge-coupled devices or switch capacitor type) is necessary for forming large infrared focal plane arrays.
Large focal plane arrays with high resistance infrared detectors is the prime motivation for the selection of photovoltaic type detectors. Photovoltaic detectors are diodes which when reversed-biased by several millivolts exhibit high resistance. Dependent upon the material quality and structure used, this resistance can increase from the 50 to 100 ohms range for photoconductive infrared detectors, to the kilo-ohm or gigaohm range for photovoltaic infrared detectors.
However, photovoltaic detectors do not have signal gain. Thus, by switching from photoconductive detectors to photovoltaic detectors we are replacing appreciable photoconductive gain with high resistance diodes having optimally unity photoconductive gain. The change from photoconductive to photovoltaic detectors is motivated by the need for an increased resistance detector which has low power consumption. However, this change on the focal is at the expense of the signal-to-noise ratio because with unity gain detectors all read-out circuit noises become more significant and deleterious.
Poor signal-to-noise ratio may result from unity gain detectors. The noise from the processor and any shunting resistance across the detector becomes more significant. To maximize the signal-to-noise ratio, infrared photodiode arrays are needed with a high R.sub.o A product shunting resistance value and low excess noise when reversed biased. The R.sub.o A product is defined as the resistance of a photovoltaic detector biased at zero volts multiplied by its junction area. The necessary R.sub.o A value can be computed from the flux incident on the detector. In the 8-11.5 .mu.m longwave infrared (LWIR) band, the photon flux from a 300K background is: EQU .PHI.=2.times.10.sup.16 photons/cm.sup.2 sec.
The required photodiode resistance (R.sub.D) for achieving background limited imaging performance can be expressed as: EQU R.sub.D .gtoreq.(4kT)/(2e.sup.2 .PHI..eta.A)
where,
.eta.=detector quantum efficiency PA1 e=electronic charge PA1 k=Boltzmann's constant PA1 T=temperature in degrees Kelvin PA1 A=the detector's area
For example, a 33 .mu.m square detector operating at 77.degree. K., the value of R.sub.D should be greater than 7.62.times.10.sup.5 ohms or, the R.sub.o A product should be greater than 8.30 ohm-cm.sup.2. Attainment of a high injection efficiency (needed for high S/N ratio) with a direct injection-detector-CCD coupling arrangement requires an even greater R.sub.o A product, i.e., greater than 20 ohm-cm.sup.2.
In the existing detector technology, attainment of photovoltaic detectors with large R.sub.o A products (in the range of approximately 20 ohm-cm.sup.2) is very difficult. For detector operation in a low background environment, the insufficient R.sub.o A problems become worse because the required R.sub.o A products increase inversely with the photon flux. A preamplifier could be connected to each photovoltaic detector to circumvent the low R.sub.o A value but this requires inclusion of an amplifier which will increase the power consumption per each detector and thus for the entire focal plane array.
It is therefore desirable to increase the output signal levels produced by the infrared sensor, as obtained with photoconductive detectors, and thereby reduce the required R.sub.o A value. Conventional infrared photoconductors with large photoconductive gain have too low a resistance value causing two problems: (1) difficulty in coupling a photoconductive detector directly to a charge coupled device focal plane signal processor; and (2) the high power consumed by an appropriate focal plane signal processor. These problems; low R.sub.o A product, poor signal-to-noise ratio, and focal plane power consumption, can be solved with a new photoconductor device that has a higher resistance, e.g., greater than 10,000 ohms.